1. Field of the Invention
The present invention relates to a technique for driving a liquid crystal display, and more particularly to a method of driving a liquid crystal display that is capable of preventing a generation of a residual image and a flicker phenomenon to improve a picture quality.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) of an active matrix driving system uses thin film transistors (TFT's) as switching devices to display a natural moving picture. Since such a LCD can be implemented into a device smaller in size than the presently existing Brown tube, it has been widely used as a monitor for personal and notebook computers as well as office automation equipment such as copy machines and portable equipment such as cellular phones and pagers.
In FIG. 1, a conventional LCD allows a liquid crystal display device to record image data on the liquid crystal cells during a display frame of 16.67 ms, thereby displaying an image, and continuously records image data in each display frame. In such a LCD device, a backlight is always turned on. In the course of recording the data in any one display frame, a liquid crystal display device also is responsive to a data voltage applied to a liquid crystal cell in the previous display frame. Accordingly, in the conventional LCD, a residual image of the previous display frame remains on the liquid crystal display screen. In other words, since a response time of the liquid crystal display exists when the current display frame is turned over into the next display frame, data of the previous display frame remains on the display screen as shown in FIG. 2, thereby deteriorating picture quality. This phenomenon presents a more serious problem in the case of a moving image.
To overcome this problem, a LCD that allows an image signal to be compensated at every display frame has been disclosed in Japanese Laid-open Patent Gazette No. 1991-212615. In this LCD device, a modified difference signal is calculated on a basis of a difference signal between fields for each display frame. Specifically, a modified difference signal is determined based on a difference signal between adjacent scanning lines and a level of an image signal. Then the modified difference signal is added to the image signal to eliminate a residual display image that would otherwise emerge upon the liquid crystal display screen.
However, since such a LCD device uses a difference signal between fields to construct a single image, i.e., a difference signal between adjacent scanning lines, the difference signal may distort the image signal. Accordingly, a distorted image may result that is different from an initial image on the liquid crystal display screen. Furthermore, in the conventional LCD, a voltage difference ΔVp is generated between a voltage Vpxl that is applied to the liquid crystal cell and an effective voltage Veff that remains in the liquid crystal cell, thereby causing a flicker phenomenon.
In FIG. 3, a pixel unit of the conventional LCD device includes a gate electrode G electrically connected to a gate line 2, a drain electrode D electrically connected to a data line 4, and a thin film transistor (TFT) 6 electrically connected to a pixel electrode PXL. The pixel unit further includes a liquid crystal cell 8 and a storage capacitor Cst disposed between the pixel electrode PXL and a common electrode Vcom.
The TFT 6 is selectively turned on by a pulse-shaped gate high voltage, as shown in FIG. 4, and electrically connects the data line 4 to the liquid crystal cell 8 and the storage capacitor Cst. The liquid crystal cell 8 and the storage capacitor Cst are charged with a data voltage VD from the data line 4 when the TFT 6 is turned on, and maintains the same voltage until the TFT 6 is again turned on (i.e., when a high voltage Vgh is applied to the gate electrode). When a voltage on the gate electrode is changed from a high voltage Vgh to a low voltage Vgl (i.e., when the TFT 6 is turned off) a voltage VLC at the liquid crystal cell decreases by ΔVp.
A voltage difference ΔVp between an effective voltage Veff remaining in the liquid crystal cell and a voltage Vpxl that is applied to the liquid crystal cell is given by the following equation:ΔVp=Cgs(Vgh−Vgl)/Cgs+Cst+Clc  (1)wherein Cgs represents a parasitic capacitance between the gate and source electrodes, Cst represents a storage capacitor value, Clc represents a capacitance of the liquid crystal cell, Vgh represents a gate high voltage, and Vgl represents a gate low voltage.
It can be seen from the above equation (1) that ΔVp is mainly dominated by the parasitic capacitance Cgs and a voltage difference (i.e., Vgh−Vgl) of the gate voltage. In a liquid crystal cell having positive and negative data voltages as shown in FIG. 4, an effective voltage Veff that remains within the liquid crystal cell becomes lower than a voltage Vpxl that is applied to the liquid crystal cell by ΔVp. In particular, since the screen has a different brightness as ΔVp becomes different for each liquid crystal cell, a flicker phenomenon occurs. A major reason for this difference in brightness is that a capacitance Clc of the liquid crystal cell becomes different for each liquid crystal cell due to an affect of the previous data when a new data is applied.